Hm. An old piano is only boring when not played properly. The moment you start breathing life into it, it will start shining like no decals can ever be. So, you know...
Oh...actually that would be a cool idea....decorating a boring old piano with decals. Notes perhaps.
One has to watch out for compliments lke: "Oh, your instrument is such a lovely looking thing", instead of "Oh, your playing was so moving".
I think Micha has a good point here.
Of course, in this world we live in, you can do anything you like. But you must be aware of how the people around you will react to it! People have a very fine sense of what is "normal", and often react cautiously if they perceive something as "unusual".
Now, musical instruments are normally things of beauty. Most of them have interesting shapes, and almost all have decoration of some sort. BUT each instrument has developed an aesthetic of its own. The beauty of a Neapolitan mandolin lies in the lavish use of ebony and mother-of-pearl, whereas the classical guitar relies on its classic shape and the contrasting woods used. Baroque harpsichords often had an allegorical painting inside the lid - the modern grand piano does not.
Much of the decoration is functional: the violin, for instance, has a scroll so that it can be hung on the wall, and purfling round the edges to prevent cracks from propagating themselves. The aesthetics are secondary.
To the concertinas: These need some sort of holes in the ends to let the air in and out, and it has become usual to provide these holes in the form of elaborate fretwork, which is a functional element executed in an decorative manner. Some cheaper German concertinas have simple holes drilled for this purpose - but the holes are usually arranged in circles, forming a sort of "rosette" motif. So the concertina has its own visual appeal, which is closely linked with its function. This is "normally" regarded as enough decoration. Adding painting or decals is, as Misha pointed out, disrespectful to the instrument and to its maker.
A car will not go faster if you stick stripes on it - but people who have tuned their cars to go faster, and have the ability to control them at high speed, get to stick stripes on them. By analogy, first learn to make your concertina sound great, and then, if you still want to, stick decals on it. Then people will say, "Her concertina looks tacky, but she's a great player!"
First tune the engine and suspension and hone your driving skills - then apply the stripes. First use your time and creativity to see that your concertina is in good shape and that you can exploit it to the full. Then you can think about superficial decorations. If you still want to ...